Method for flame location transition from a start-up location to a perforated flame holder

ABSTRACT

According to an embodiment, a combustion system is provided, which includes a nozzle configured to emit a diverging fuel flow, a flame holder positioned in the path of the fuel flow and that includes a plurality of apertures extending therethrough, and a preheat mechanism configured to heat the flame to a temperature exceeding a startup temperature threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation In Part of PCT PatentApplication No. PCT/US2014/016622, entitled “STARTUP METHOD ANDMECHANISM FOR A BURNER HAVING A PERFORATED FLAME HOLDER”, filed Feb. 14,2014 (Agency docket number 2651-204-04), co-pending herewith at the timeof filing; which application claims priority benefit of U.S. ProvisionalPatent Application No. 61/765,022, entitled “PERFORATED FLAME HOLDER ANDBURNER INCLUDING A PERFORATED FLAME HOLDER”, filed Feb. 14, 2013 (Agencydocket number: 2651-172-02); and U.S. Provisional Patent Application No.61/931,407, entitled “LOW NOx FIRE TUBE BOILER”, filed Jan. 24, 2014(Agency docket number 2651-205-02); each of which, to the extent notinconsistent with the disclosure herein, is incorporated by reference.

The present application is related to PCT Patent Application No.PCT/US2014/016628, entitled “PERFORATED FLAME HOLDER AND BURNERINCLUDING A PERFORATED FLAME HOLDER”, filed Feb. 14, 2014 (Agency docketnumber 2651-172-04), co-pending herewith at the time of filing; PCTPatent Application No. PCT/US2014/016632, entitled “FUEL COMBUSTIONSYSTEM WITH A PERFORATED REACTION HOLDER”, filed Feb. 14, 2014 (Agencydocket number 2651-188-04), co-pending herewith at the time of filing;and PCT Patent Application No. PCT/US2014/016626, entitled “SELECTABLEDILUTION LOW NOx BURNER”, filed Feb. 14, 2014 (Agency docket number2651-167-04), co-pending herewith at the time of filing.

BACKGROUND

Combustion systems are widely employed throughout society. There is acontinual effort to improve the efficiency and reduce harmful emissionsof combustion systems.

SUMMARY

According to an embodiment, a combustion system is provided thatincludes a fuel nozzle configured to emit a diverging fuel flow, aperforated flame holder positioned in the path of the fuel flow andincluding a plurality of apertures extending therethrough. Thecombustion system also includes a preheater for preheating theperforated flame holder.

According to an embodiment, a combustion system includes a fuel nozzleconfigured to emit a diverging fuel flow, a perforated flame holderincluding a plurality of apertures extending therethrough positioned inthe path of the fuel flow, and a preheating means for preheating theperforated flame holder.

According to an embodiment, the preheater and preheating means includesan electrically resistive element.

According to respective embodiments, the preheater and the preheatingmeans include electrically inductive elements.

According to another embodiment, the preheater and preheating meansincludes a preheat nozzle configured to support a preheat flame in aposition between the fuel nozzle and the flame holder.

According to an embodiment, preheating means includes first and secondelectrodes configured to hold a preheat flame in the position betweenthe primary nozzle and the flame holder.

According to another embodiment, the preheating means includes a preheatflame holder configured to support a preheat flame in the diverging fuelflow in a position between the primary nozzle and the flame holder.

According to an embodiment, a combustion system is provided, whichincludes a nozzle configured to emit a diverging fuel flow, a flameholder positioned in the path of the fuel flow and that includes aplurality of apertures extending therethrough. The combustion systemalso includes preheating means, for preheating the primary flame holder.

According to an embodiment, a method of operation is provided, foroperation of a flame holder. The method includes performing a burnerstartup procedure, including applying thermal energy to the flameholder, terminating the burner startup procedure after a temperature ofa portion of the flame holder is above a startup temperature threshold,emitting a flow of fuel from a nozzle at an operational rate, andsupporting a flame within a plurality of apertures extending through theflame holder. According to an embodiment, the applying thermal energyincludes generating heat by applying an electrical potential across anelectrically resistive element, and applying the heat to the flameholder.

According to an embodiment, the performing a burner startup procedurefurther includes holding a flame supported by fuel emitted from theprimary nozzle at a location between the primary nozzle and a side ofthe flame holder facing the primary nozzle.

According to an embodiment, the performing a burner startup procedureincludes holding the primary nozzle in a startup position and emitting aflow of fuel from the primary nozzle at a startup rate, the startup ratebeing sufficiently low as to enable a stable flame within the fuel flow.Additionally, the terminating the burner startup procedure includesmoving the primary nozzle from the startup position to an operationalposition.

According to an embodiment, the terminating the burner startup procedureincludes reducing a concentration of oxygen in a flow of air.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic perspective view of a burner system, accordingto an embodiment.

FIGS. 2A and 2B are diagrammatic side sectional views of a combustionsystem in respective modes of operation, according to an embodiment.

FIG. 3 is a diagrammatical side view of a combustion system, accordingto an embodiment, portions of which are shown in sectional view.

FIG. 4 is a diagrammatic side sectional view of a combustion system,according to an embodiment.

FIG. 5 is a diagrammatic perspective view of a combustion system,according to an embodiment.

FIGS. 6-9 are diagrammatic cross sections of flame holders, according torespective embodiments.

FIG. 10 is a diagrammatic cross section of a portion of a boiler thatincludes a combustion system, according to an embodiment.

FIG. 11A is a diagrammatic cross section of a combustion system duringnormal operation, according to an embodiment.

FIG. 11B is a diagrammatic cross section of a combustion system duringstartup, according to an embodiment.

FIG. 12 is a diagrammatical side sectional view of a combustion system,according to an embodiment.

FIGS. 13-18 are flow charts illustrating methods of operation of aburner system, according to respective embodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description and drawings are not meant to be limiting. Otherembodiments may be utilized, and other changes may be made, withoutdeparting from the spirit or scope of the subject matter presented here.

FIG. 1 is a diagrammatic perspective view of a burner system 100,according to an embodiment. The burner system 100 includes a nozzle 102and a flame holder 104 positioned within a combustion volume 106, partof which is defined by a wall 108. An aperture 110 extends through thewall 108. The nozzle 102 extends into the combustion volume 106 via theaperture 110. The aperture 110 is of a size selected to permit air toenter the combustion volume 106 in order to provide oxygen to supportcombustion within the combustion volume 106.

The term air refers to a fluid, typically gaseous, that includes oxygenin a form that can support combustion. While ambient air is the mostcommon oxidizer fluid, many combustion systems employ a mixture that caninclude other additives, selected, generally, for the purpose ofmodifying the combustion in some way. For example, in many cases, fluegases are recirculated and mixed with ambient air to reduce the oxygenconcentration in the oxidizer fluid, in order to reduce harmfulemissions, according to very well known principles and processes. Forthe purpose of the present specification and claims, the term air is tobe construed as referring to any such fluid or mixture of fluids, unlessexplicitly limited or defined otherwise.

The flame holder 104 of the combustion system 100 is preferably madefrom a refractory material, such as, e.g., ceramic, and includes aplurality of apertures 112, that extend from a first face 114 to asecond face 116 of the flame holder.

The flame holder is positioned with the first face 114 toward the nozzle102 and spaced away from the nozzle by a distance D₁. In the embodimentshown, the first and second faces 114, 116 of the flame holder 104 areplanar, and lie substantially parallel to a plane that is perpendicularto a longitudinal axis A of the nozzle 102. According to alternativeembodiments, the flame holder 104 can include faces that are non-planar,or that lie at different angles with respect to each other or to thelongitudinal axis A of the nozzle 102.

In operation, a stream of fuel 118 is ejected from the nozzle 102 towardthe first face 114 of the flame holder 104. The fuel stream 118disperses from the nozzle 102 in a conical spray at an angle that istypically about 7.5 degrees from the longitudinal axis A, resulting in asolid conical angle of about 15 degrees. As the fuel stream 118disperses, it entrains air, and eventually reaches a flammableproportion of fuel and air. By selection of the nozzle 102 and thepressure at which fuel is ejected, the velocity at which the fuel stream118 is ejected from the nozzle 102 is preferably selected to be muchhigher than the flame propagation speed of the particular type of fuelemployed, so that, on the one hand, the fuel stream is prevented fromsupporting a flame near the nozzle 102, and on the other hand, by thetime the dispersing fuel stream 118 has slowed to near the flamepropagation speed, the fuel stream has entrained enough air that themixture is too lean for combustion at the temperature of the fuel stream218.

However, the flame holder 104 is held at a much higher temperaturebecause of ongoing combustion. The higher temperature of the flameholder 104 is sufficient to maintain combustion of the lean fuelmixture. A stable flame can thus be maintained by the flame holder 104.The flame is held primarily within the apertures 112, although the flamemay extend a short distance beyond either or both faces 114, 116 of theflame holder 104. The fuel stream 118 is able to continually feed thecombustion, and the flame holder 104 is able to support a leaner flamethan could be maintained in a conventional burner system. The distanceD₁ is selected, in part, according to a desired fuel-to-air ratio of thefuel stream 118 at the point at which the fuel stream contacts the flameholder 104.

The inventors have recognized that, although while in operation, theflame holder 104 is able to support combustion with a very lean fuelmixture, startup of the combustion system 100 is problematic. Becausethe fuel mixture at the distance D₁ is flammable only at elevatedtemperatures, and because the flame holder 104 may be at ambienttemperature at startup, conventional ignition methods or devices are notgenerally effective for startup of the combustion system 100.

The inventors have developed various systems and methods for efficientstartup of combustion systems that employ perforated flame holders. Inthe embodiments described hereafter, elements that are described withreference to previous embodiments, and that are referenced by a samereference number, are generally not described in detail.

FIGS. 2A and 2B are diagrammatic side sectional views of a combustionsystem 120, according to an embodiment. The system 120 includes a nozzle122, a nozzle position controller 124, a system control unit 126, a fuelsupply 128, a fuel control valve 130, and a temperature sensor 132.

The nozzle 122 is configured to be translatable along its longitudinalaxis A. As shown, the nozzle 122 includes inner and outer telescopingelements 134, 136. The inner telescoping element 134 is fixed, while theouter telescoping element 136, which includes a nozzle orifice 138, ismovable between a retracted position, as shown in FIG. 2A, and anextended position, which is shown in FIG. 2B and described in moredetail below. Movement of the outer telescoping element 136 changes thedistance between the nozzle 122 and the flame holder 104. Movement ofthe outer telescoping element 136 is controlled by operation of thenozzle position controller 124, which, in the example shown, acts as alinear actuator driving a position control rod 140 that is coupled tothe outer telescoping element 136.

The fuel supply 128 is coupled to the nozzle via a fluid line 142. Thefuel control valve 130 is coupled in the fluid line 142 and configuredto control the flow of fuel from the fuel supply 128 to the nozzle 122.

The temperature sensor 132 is positioned and configured to detect atemperature of the flame holder 104, and to transmit a signalrepresentative of the detected temperature to the system control unit126 via connector 144. As shown in FIG. 2A, the temperature sensor 132is an infrared transducer, configured to detect infrared emissions ofthe flame holder 104 and to interpret the detected emissions to producea signal that is representative of the temperature. Other types oftemperature sensors are well known that can be used to detect thetemperature of the flame holder 104. These include, for example, sensorsbased on thermocouples, thermistors, pyrometers, radiometers, positiveor negative thermal coefficient resistors, etc.

The system control unit 126 is operatively coupled to the temperaturesensor 132, the fuel control valve 130, and the nozzle positioncontroller 124 via connectors 144, and is configured to receive datafrom the temperature sensor 132 and to control operation of the fuelcontrol valve 130 and the nozzle position controller 124.

The combustion system 120 is shown in FIG. 2A in an operational mode,operating substantially as described with reference to the system 100 ofFIG. 1. In operational mode, the nozzle 122 is separated from the firstface 114 of the flame holder 104 by the distance D₁, a fuel stream 118is ejected from the nozzle 122 at an operational rate, and the flameholder 104 supports a stable primary flame 146.

FIG. 2B shows the combustion system 120 in startup mode, in which theouter telescoping element 136 is in its extended, i.e., startupposition, in which the distance D₂ between the nozzle 122 and the flameholder 104 is significantly reduced, as compared to the distance D₁.Additionally, the system control unit 126 controls the fuel controlvalve 130 to reduce the volume and velocity of the fuel stream 118ejected by the nozzle 122. Because the velocity of the fuel stream 118is reduced, a stable startup flame 148 can be supported by the nozzle122, alone, in a position between the nozzle and the flame holder 104.By moving the nozzle 122 to the extended position, the startup flame 148is positioned close to the flame holder 104, and is thus able to quicklyheat a portion of the flame holder 104 to a temperature that exceeds athreshold defining a minimum startup temperature (i.e., the startuptemperature threshold) of the flame holder. When the signal from thetemperature sensor 132 indicates that the temperature of the flameholder 104 is above the threshold, the system control unit 126 controlsthe nozzle position controller 124 to move the nozzle 122 to theretracted, operational position, and controls the fuel control valve 130to open further, increasing the fuel flow 118 to an operational level.As the velocity of the fuel stream 118 increases, the startup flame 148is blown out. As the uncombusted fuel mixture reaches the flame holder104, the mixture auto-ignites, at least within the portion of the flameholder that has been heated beyond the startup threshold. Very quicklythereafter, the entire flame holder 104 is heated to its operatingtemperature, and continues in normal operation thereafter.

According to another embodiment, the system control unit 126 includes atimer by which transition from startup mode to operational mode iscontrolled. i.e., when startup is initiated, the system control unit 126starts the timer, and when a selected time period has passed, the nozzle122 is retracted and the fuel flow is increased, as described above. Thetime period is selected according to a predetermined period necessary toensure that the flame holder 104 has reached the startup temperaturethreshold.

The movable nozzle 122 can also be employed in combustion systems thatmay be required to operate on a variety of fuels. As is well known inthe art, the fuel-to-air ratio at which the mixture is combustiblevaries according to the type of fuel, as does flame propagation speedwithin a flow of fuel. Thus, an optimal operating distance D₁ will varyaccording to the type of fuel. The combustion system 120 of FIGS. 2A and2B can accommodate changes in fuel type by adjustment of the position ofthe nozzle 122 relative to the flame holder 104. The adjustment can bemade by direct manual control of the nozzle 122, or the system controlunit 126 can be programmed to make the adjustment automatically. Forexample, additional sensors can be positioned to detect emission levelsof flames propagating within the fuel stream 118, incomplete combustion,etc., in response to which the system control unit 126 can be programmedto modify the position of the nozzle 122 and/or the fuel flow (byadjustment of the fuel control valve 130, to bring the operation of thesystem closer to an optimum or desired level.

According to an alternate embodiment, the flame holder 104 is configuredto be translatable along the longitudinal axis A of the nozzle. Thisenables adjustments in the distance D between the nozzle 122 and flameholder 104 without changing the position of the nozzle. According to anembodiment, the combustion system 120 includes a flame holder that istranslatable along the axis A and also a nozzle that is movable ortranslatable along the axis A. With such a configuration, a user can,for example, move the flame holder to a position that is more or lesspermanent, in order to establish a desired distance D on the basis ofcombustion characteristics of a particular fuel. Meanwhile, the nozzlecan be configured to be moved for startup purposes, as described above,or for some other reason.

FIG. 3 is a diagrammatical side view of a combustion system 150,according to an embodiment, portions of which are shown in section. Thecombustion system includes a first electrode 152 and preferably a secondelectrode 154, both operatively coupled to a voltage supply 158. Anelectrode position controller 156 and the voltage supply 158 are alsocoupled to a system control unit 126.

The first electrode 152 is in the shape of a torus, positioned justdownstream of the nozzle 102 and centered on the longitudinal axis A ofthe nozzle so that the fuel stream 118 passes through the firstelectrode 152. The second electrode 154 is positioned between the firstface 114 of the flame holder 104 and the nozzle 102. The secondelectrode 154 is movable from an extended position, as shown in solidlines in FIG. 3, to a retracted position, shown in phantom lines. Theelectrode position controller 156 is configured to extend and retractthe second electrode 154. In the extended position, the second electrode154 extends to a position close to or intersecting the longitudinal axisA. In the retracted position, the second electrode 154 is spaced awayfrom contact with the fuel stream 118 or a flame supported thereby.According to an embodiment, a temperature sensor 132 is provided, aspreviously described.

In operation, when the combustion system 150 is in startup mode, i.e.,when startup is initiated, the system control unit 126 controls theelectrode position control 156 to move the second electrode 154 to theextended position. The system control unit 126 controls the voltagesupply 158 to transmit a first voltage signal to the first electrode152. As the fuel stream 118 passes through the first electrode 152, anelectrical charge having a first polarity is imparted to the fuelstream. Meanwhile, the system control unit 126 transmits a secondvoltage signal from the voltage supply 158 to the second electrode 154.The second voltage signal has a polarity that is opposite that of thecharge imparted to the fuel stream, and therefore attracts theoppositely-charged fuel stream. Ignition is initiated within the fuelstream 118, whereupon a startup flame 148 is held between the first andsecond electrodes 152, 154, in spite of the high velocity of the fuelstream. This method of holding a flame within a fuel flow is sometimesreferred to as electrodynamic combustion control.

According to an embodiment, the system control unit 126 controls thevoltage supply 158 to apply a voltage signal to the second electrode 154while connecting the first electrode 152 to ground. According to anembodiment, the voltage signal applied to the first and/or secondelectrode is an AC signal.

With the startup flame 148 held below the first surface 114 of the flameholder 104, a portion of the flame holder 104 is quickly heated to thestartup temperature threshold. When the startup temperature threshold issurpassed, the system control unit 126 controls the voltage supply 158to remove the voltage signals from the first and second electrodes 152,154, and controls the electrode position controller 156 to move thesecond electrode 154 to the retracted position. When the voltage signalsare removed from the electrodes 152, 154, the startup flame 148 is nolonger held, and blows out. As previously described, when theuncombusted fuel and air mixture reaches the flame holder 104, theprimary flame auto-ignites in the preheated portions of the flameholder, and normal operation quickly follows.

Although embodiments are described as including a system control unit126 that is configured to control transition between a startup mode andan operational mode, alternative embodiments are operated manually. Forexample, according to an embodiment, the combustion system 150 isconfigured such that an operator manually switches the electrodeposition controller 156 to move the second electrode 154. According toanother embodiment, the operator manually extends and retracts thesecond electrode 154. Additionally, according to an embodiment, anoperator manually switches a voltage signal to the first and secondelectrodes 152, 154, and switches the signals off when the flame holder104 exceeds the startup threshold.

According to an embodiment, a portion of the nozzle 102 is electricallyconductive, and acts as the first electrode 152, with the connector 144being coupled to the conductive portion of the nozzle. According to anembodiment, the second nozzle 154 is retracted by a telescopingmechanism similar to that described with reference to the nozzle ofFIGS. 2A and 2B. According to another embodiment, the second electrode154 is formed as a conductive layer positioned on the first face 114 ofthe flame holder 104.

FIG. 4 is a diagrammatic side sectional view of a combustion system 160,according to an embodiment. In the combustion system 160, the nozzle 102is a primary nozzle, and the system further includes a secondary nozzle162 positioned between the primary nozzle and the flame holder 104. Thefuel supply 158 is coupled to the primary nozzle 102 and the secondarynozzle 162 via fuel lines 142. A primary fuel valve 130 controls a flowof fuel from the fuel supply 158 to the primary nozzle 102, and asecondary fuel valve 164 controls a flow of fuel from the fuel supply tothe secondary nozzle 162. The system control unit 126 is operativelycoupled to the primary and secondary fuel valves 130, 164 via connectors144.

In operation, when startup is initiated, the system control unit 126controls the secondary fuel valve 164 to open—the primary fuel valve 130is closed—and ignites a stream of fuel that exits the secondary nozzle162, producing a startup flame 148 that is directly adjacent to thefirst face 114 of the flame holder 104. The startup flame 148 heats aportion of the flame holder 104 to a temperature exceeding the startupthreshold. When the system control unit 126 determines that a portion ofthe flame holder 104 exceeds the startup temperature threshold—via, forexample, a signal from a temperature sensor, as described previously—thesystem control unit controls the secondary fuel valve 164 to close,while controlling the primary fuel control valve 130 to open, causing afuel stream 118 to be ejected by the primary nozzle 102. When the fueland air mixture of the fuel stream 118 reaches the flame holder 104, aprimary flame is ignited and normal operation follows, substantially asdescribed with reference to previously embodiments.

FIG. 5 is a diagrammatic perspective view of a combustion system 170,according to an embodiment. The burner system 170 is similar in manyrespects to the system 100 described with reference to FIG. 1, andincludes many of the same elements. However, the system 170 alsoincludes an electrically resistive heating element 172. In theembodiment shown, the heating element 172 is in the form of a wire thatis interleaved in and out through some of the plurality of apertures112. The heating element 172 is operatively coupled to a voltage supply158 via a connector 144. During a startup procedure, the system controlunit 126 controls the voltage supply 158 to apply a voltage potentialacross the ends of the heating element 172. The resistance value of theheating element 172 and the magnitude of the voltage potential areselected to generate sufficient heat to raise the temperature of theportion of the flame holder 104 in the vicinity of the heating elementto beyond the startup threshold within a few seconds, after which thesystem control unit 126 controls valve 130 to open, while controllingthe voltage supply 158 to remove the voltage potential from the heatingelement 172. When the fuel stream 118 contacts the heated portion of theflame holder 104, auto-ignition occurs, and a stable flame isestablished in the flame holder 104. Thereafter, operation of the burnersystem 170 is substantially as described previously with reference toother embodiments.

FIGS. 6-9 are diagrammatic cross sections of flame holders according torespective embodiments. Each of the flame holders of FIGS. 6-9 includesan electrically resistive heating element that is integral with therespective flame holder. Each of the flame holders is configured for usein a combustion system similar to the system 170 described withreference to FIG. 5.

FIG. 6 shows a flame holder 180 that includes a heating element 182 thatis encapsulated by the material of the flame holder. Terminals 184 areelectrically coupled to the heating element 182 and extend from theflame holder 180 for connection to a voltage source. According to anembodiment, the heating element 182 extends across the entire flameholder 180, so that, in operation, the entire flame holder is heatedwhen the heating element is energized. Thus, when the flame holder 180is heated to beyond the startup threshold and a fuel and air mixture isturned on, the entire flame holder supports combustion instantly.According to another embodiment, the heating element 182 extends acrossonly a portion of the flame holder 180. In this embodiment, less poweris required to bring the portion of the flame holder 180 to the startuptemperature threshold, and the heating element 182 takes less time toheat the smaller portion of the flame holder 180.

FIG. 7 shows a flame holder 190 that includes a heating element 192located on one of the faces of the flame holder. This embodiment may besimpler to manufacture than the flame holder of FIG. 6, inasmuch as themanufacturing options are wider. For example, the heating element 192can be attached to the flame holder 190 at the end of the manufacturingprocess, or it can be attached at an intermediate step, then shaped ormachined along with the main part of the flame holder.

FIG. 8 shows a flame holder 200 that includes a section of thermallyconductive material 202 that is encapsulated in the material of the mainpart of the flame holder, and a heating element 204 coupled to the flameholder in direct thermal contact with the section of thermallyconductive material. In operation, when the heating element 204 isenergized, heat is transmitted by conduction to the section of thermallyconductive material 202, which carries the heat into a portion of theflame holder 200. One advantage of this embodiment is that the heatingelement can be configured to be removable,

which enables, in the event of a malfunction, replacement of the heatingelement 204 alone, instead of the entire flame holder 200.

FIG. 9 shows a flame holder 210 that is made from a material thatincludes particles of an electrically conductive substance. For example,the flame holder 210 can be made of a ceramic material impregnated withmetallic particles. By selecting the density of conductive particles,the resistance of the material can be selected. The terminals 184 arecoupled to conductive plates 212 in direct contact with the material ofthe flame holder 210. The plates 212 provide a broad electrical contactwith the material in order to avoid highly resistive conduction regions,which would create hot spots at the points of contact.

FIG. 10 is a diagrammatic cross section of a portion of a boiler 220,according to an embodiment. The boiler 220 includes a burner mechanism222 and a startup apparatus 224. The burner mechanism includes a nozzle102 and a primary flame holder 104. The startup apparatus 224 includes astartup flame holder 226 and a flame holder position controller 228. Thestartup flame holder 226 includes a bluff body configured to causevortices to circulate heat to maintain a start-up flame 148, in order topre-heat the primary flame holder 104 during a startup procedure. Theflame holder position controller 228 is configured to move the startupflame holder 226 between a startup position, as shown in FIG. 10, and aretracted position, as shown in phantom lines. The flame holder positioncontroller 228 includes a rod (also indicated by 228) that extendsthrough a wall 108 for access from outside the combustion volume 106. Inthe embodiment shown, the flame holder position controller 228 isconfigured to be manipulated by an operator before and after a startupprocedure. According to an alternate embodiment, the flame holderposition controller 228 includes an actuator that automatically movesthe startup flame holder 226 between the startup position and theretracted position.

To move the startup flame holder 226 from the startup position to theretracted position, the operator grasps a handle 230 at the end of therod 228, rotates the rod, which moves the startup flame holder 226substantially out of the path of the fuel stream 118, then pulls the rodoutward, which translates the startup flame holder into a space that isfully out of the path of the fuel stream 118.

FIG. 11A is a diagrammatic cross section of a combustion system 240during normal operation, according to an embodiment. The combustionsystem 240 includes a flame holder 104, a nozzle 102, an air conduit 242and an air mixture control valve 244. The air conduit 242 is coupled tothe wall 108 over the aperture 110. Thus, air 246 entering thecombustion volume 106 passes through the air conduit 242.

The air mixture control valve 244 includes a valve gate 247 and acts asa proportion control valve, configured to control a proportion ofcomponents of the air 246 that is introduced into the combustion volume106. In the embodiment shown, the air mixture control valve 244 isconfigured to control the respective proportions of two components thatare introduced via first and second valve input ports 248, 250. Pivotingof the valve gate 247 in, for example, a clockwise direction increasesthe proportion of a fluid entering via the first valve input port 248while simultaneously reducing the proportion of fluid entering the valvevia the second valve input port 250. Conversely, pivoting of the valvegate 247 in the counter-clockwise direction reduces the proportion fromthe first valve input port 248 and increases the proportion from thesecond valve input port 250.

In the embodiment of FIG. 11A, recirculated flue gas 252 is introducedto the control valve 244 via the first input port 248, while ambient air254 is introduced via the second input port 250. The valve gate 247controls the proportions of the recirculated flue gas 252 and ambientair 254 that are admitted to air conduit 242 via the valve 244.

As explained in more detail above with reference to FIG. 1, incombustion systems of the general type described in the presentdisclosure, nozzle shape and fuel pressure are preferably selected toprevent the fuel stream 118 from supporting a flame separate from theflame holder 104. Specifically, the fuel stream velocity at the nozzle102 is selected to be sufficiently high that by the time the stream 118has slowed to about the flame propagation speed, for the particular typeof fuel, the fuel in the stream will have been diluted by entrained airto the point that it is no longer flammable at the temperature of thefuel stream 118. This point is related not only to the amount of fueland oxygen in the mix, but also to the ratio of fuel to other componentsin the stream. Dilution of the fuel in the stream 118 separates the fuelmolecules so that combustion of one molecule does not provide sufficientheat to nearby fuel molecules to maintain combustion. Thus, in somecases, the fuel-air mix in the fuel stream 118 can be made flammablemore quickly if the proportion of oxygen in the air is higher.

According to an embodiment, the combustion system 240 is configured fornormal operation with air that has a significantly lower oxygen contentthan is found in ambient air. Accordingly, as shown in FIG. 11A, valvegate 247 of the air mixture valve 244 is positioned to produce an airmixture that has a high proportion of recirculated flue gas 252, whichhas a relatively low oxygen content, thereby reducing the overall oxygenconcentration in the air.

While the introduction of recirculated flue gas can have beneficialeffects with respect to the production of NOx, a system configured tooperate with recirculated flue gas can have advantages related tostartup, as well.

FIG. 11B shows the combustion system 240 in a startup mode, according toan embodiment. It can be seen that the valve gate 247 is in a positionin which the first input port 248 is completely closed, meaning thatthere is no recirculated flue gas 252 being introduced. Consequently,ambient air 254 is the only component of the air 246 that is introducedinto the combustion volume 106 during startup. The result is that theoxygen concentration in the entrained air is higher than would beappropriate for normal operation, and that the fuel stream 118 is stillflammable at the point at which the stream 118 slows to the flamepropagation speed. Under these circumstances, the fuel stream 118 cansupport a startup flame 148 at a position between the nozzle 102 and theflame holder 104. The system 240 operates in startup mode long enoughfor the flame holder 104 to be heated by the startup flame 148. Once atleast a portion of the flame holder 104 exceeds the startup threshold,the control gate 247 is controlled to move toward the position shown inFIG. 11A, introducing recirculated flue gas 252, and reducing the oxygenconcentration of the air 246. The startup flame 148 goes out, andcombustion initiates in the apertures 112 of the flame holder 104.

In the embodiment described with reference to FIGS. 11A and 11B, astartup flame 148 is enabled by retaining flammability of the fuel/airmixture beyond a point where the fuel stream 118 has slowed to the flamepropagation speed. According to another embodiment, a startup flame 148is enabled by temporarily increasing the flame propagation speed. Forexample, flame propagation speed in hydrogen is several times that of aflame in methane. Accordingly, a system can be designed and configuredin which the velocity of the fuel stream is selected to prevent a flamefrom being supported away from the flame holder 104 while methane isejected from the nozzle, but to support a startup flame while hydrogenis provided as the fuel. While hydrogen is used, the flame propagationspeed is high enough for a flame to be supported between the nozzle andthe flame holder, but once the flame holder 104 is above the startuptemperature threshold, the fuel is switched to methane or the like,causing the flame to lift to the flame holder 104.

Various methods and devices can be used to enable operation of thecombustion system 240. For example, an oxygen sensor can be positionedin the air conduit 242, with an output coupled to the controller 126, toenable the controller to monitor the air oxygen level of the air 246. Atemperature sensor and/or a timer can be provided, as previouslydescribed, to enable the controller 126 to determine when to transitionfrom the startup mode to the operation mode, etc.

FIG. 12 is a diagrammatical side view of a combustion system 270,according to an embodiment. The combustion system 270 includes a laseremitter 272 supported by a bracket 274, positioned and configured toemit a laser beam 276 that impinges in a portion of the first face 114of a flame holder 104. Photonic energy delivered by the laser beam 276is converted into thermal energy within the flame holder 104, therebyheating a portion of the flame holder 104. When the portion of the flameholder 104 exceeds the startup temperature threshold, fuel is sent to anozzle 102 and ejected into a fuel stream 118 toward the flame holder104, and the laser 272 is shut down. In the embodiment shown, the laser272 is held in a fixed position that is sufficiently removed from theflame holder 104 and fuel stream 118 as to cause no interference withnormal operation of the system, and to be substantially unaffected bythe environment. According to another embodiment, the bracket 274 isconfigured to hold the laser emitter 272 much closer to the first face114 of the flame holder 104 for more efficient energy transfer.Accordingly, the bracket 274 is also configured to retract the laser 272from the vicinity of the fuel stream 118 when the system 270 is not instartup mode.

FIG. 12 shows a laser emitter 272 configured to transmit energy in anon-thermal form, which is converted to thermal energy upon impinging onthe flame holder 104. According to various embodiments, other devicesare configured to transmit non-thermal energy onto the flame holder 104to be converted to thermal energy. For example, according to anembodiment, a microwave transmitter is positioned and configured todirect a microwave emission onto a surface of the flame holder 104. Inthat embodiment, the flame holder 104 includes a patch of material thatis configured to absorb the microwave emission and to convert a portionof the transmitted energy to heat.

FIGS. 13-18 are flow charts illustrating methods of operation of aburner system, according to respective embodiments. FIG. 13 shows amethod 280 that includes two basic steps. In step 282, a burner startupprocedure is performed, after which the combustion system moves tonormal burner operation in step 284. The burner startup procedure ofstep 282 includes step 286, heating a flame holder to exceed a startuptemperature threshold. The normal burner operation of step 284 includesthe steps of emitting fuel at an operational rate, at step 288, andsupporting a flame in apertures of a flame holder, at step 290.

FIG. 14 illustrates a method 300 in which the heating a flame holderstep 286 of FIG. 13 includes a step 302, applying energy to the flameholder, after which, in step 304, it is determined whether the flameholder temperature is greater than the temperature threshold. If theflame holder temperature is not greater than the temperature threshold,the process returns to the previous step, 302. If the flame holdertemperature is greater than the temperature threshold, the processproceeds to normal burner operation, in step 284.

The step of applying energy to the flame holder, step 302, can beaccomplished via any of a number of different structures and devices,some examples of which are disclosed above. For example, the energy canbe applied by means of an electrically resistive element, as describedwith reference to FIGS. 5-9, by transmitting energy in a non-thermalform, as described with reference to FIG. 12, or by supporting a startupflame in a position to heat the flame holder, as described withreference to FIGS. 2B-4, 10, and 11B.

FIG. 15 illustrates a method 310 in which the heating a flame holderstep 286 of FIG. 13 includes a step 312, initializing a counter. In acycling process like that of the embodiment of FIG. 15, a counter actsas a timer element. The counter is initialized by setting it to zero,and a threshold number representing a selected time period isestablished. Following initialization of the counter in step 312, theapply energy step 302 is performed, after which a number held by thecounter is increased by one. In step 316, the count, i.e., the number inthe counter, is compared to the count threshold (CT), i.e., thethreshold number representing the selected time period. If the countdoes not exceed the count threshold, the process cycles back to theapply energy step (302) and repeats. If the count does exceed the countthreshold, the process proceeds to normal burner operation, in step 284.

FIG. 16 illustrates a method 320 in which the burner startup procedurestep 282 of FIG. 13 includes step 322, holding a startup flame in astartup position, followed by the heat flame holder step 286, then bystep 324, in which the startup flame is deactivated. Examples ofstructures configured to hold startup flames are disclosed above withreference to FIG. 2B, in which fuel is ejected from a primary at a lowervelocity, FIG. 3, in which an electrodynamic combustion control systemis employed to hold a startup flame between the primary nozzle and theflame holder, FIG. 4, in which a startup nozzle is configured to supporta startup flame, FIG. 10, in which a startup flame holder is positionedwithin a fuel stream to mechanically disrupt the fuel stream and enableheat recirculation within a turbulence zone, and FIG. 11B, in which afuel stream is made to entrain air with a higher oxygen content in orderto create a region of the primary fuel stream that will support astartup flame.

In each of the above-referenced examples, deactivation procedures arealso disclosed.

FIG. 17 illustrates a method 330 that is directed in more detail to amethod corresponding, for example, to the structure described withreference to FIGS. 2A and 2B, in a distance D between a nozzle and flameholder is reduced (step 332), fuel is emitted from a nozzle at a rateselected to support a startup flame (step 334) and to heat the flameholder (step 286) following which the distance D is increased to anoperational distance (step 336).

FIG. 18 illustrates a method 340 that is directed in more detail to amethod corresponding, for example, to the structure described withreference to FIG. 4, in which a startup nozzle is configured to supporta startup flame (step 342) and is positioned adjacent to the flameholder so as to heat the flame holder (step 286). The startup flame ofFIG. 4 is deactivated after the flame holder is heated (step 344).

According to an embodiment, a combustion system includes an oxidantsource configured to output air into a combustion volume, a primary fuelnozzle configured to output fuel into the combustion volume, and aprimary flame holder. The primary flame holder includes an input faceproximal to the primary fuel nozzle, an output face distal from theprimary fuel nozzle, and a plurality of apertures extending between theinput and output faces. The combustion system further includes a preheatmechanism configured to support a flame in a first position selected topreheat the primary flame holder, the preheat mechanism being configuredto remove the flame from the first position after preheating the primaryflame holder, the primary flame holder being configured to support acombustion reaction of the fuel and oxidant within the plurality ofapertures after being preheated by the first flame.

According to an embodiment, the first position is between the primaryfuel nozzle and the primary flame holder.

According to an embodiment, the preheat mechanism includes an actuatorcoupled to the primary fuel nozzle and configured to move the fuelnozzle between a preheat position and an operating position. The primaryfuel nozzle outputs a preheat flow of the fuel for the flame in thefirst position when the primary fuel nozzle is in the preheat position.The primary fuel nozzle outputs an operating flow of the fuel for thecombustion reaction supported by the primary flame holder when the fuelnozzle is in the operating position.

According to an embodiment, the preheating mechanism includes asecondary flame holder configured to hold the flame in the firstposition. The preheating mechanism is configured to support the flame inthe first position by moving the secondary flame holder to a preheatposition and to remove the flame from the first position by moving thesecondary flame holder from the preheat position.

According to an embodiment, the preheating mechanism is configured tosupport the flame in the first position by applying a selected voltagebetween the flame and the secondary flame holder.

According to an embodiment, the preheating mechanism can be configuredto remove the flame from the first position by terminating the flame.Alternatively, the preheating mechanism can remove the flame from thefirst position by moving the flame to a second position.

According to an embodiment, a method includes preheating a primary flameholder by supporting a flame at a first position between a primary fuelnozzle and the primary flame holder and removing the flame from thefirst position after the primary flame holder has been preheated to athreshold temperature. The method further includes outputting fuel fromthe primary fuel nozzle onto the primary flame holder and supporting acombustion reaction of the fuel within the primary flame holder afterthe primary flame holder has been preheated to the thresholdtemperature.

According to an embodiment, removing the flame from the first positionincludes terminating the flame.

According to an embodiment, supporting the flame at the first positionincludes holding the flame with a secondary flame holder.

According to an embodiment, the preheating mechanism is configured tosupport the flame in the first position by moving the secondary flameholder to a preheat position and to remove the flame from the firstposition by moving the secondary flame holder from the preheat position.

According to an embodiment, the method further includes supporting theflame in the first position by moving the primary fuel nozzle to apreheat position and outputting from the fuel nozzle a preheat flow ofthe fuel for the flame in the first position and outputting fuel fromthe primary fuel nozzle onto the primary flame holder by moving theprimary fuel nozzle from the preheat position to an operating positionand outputting from the fuel nozzle an operating flow of the fuel.

Structures configured to electrically connect components or assembliesshown in the drawings are depicted generically as connectors 144,inasmuch as electrical connectors and corresponding structures are verywell known in the art, and equivalent connections can be made using anyof a very wide range of different types of structures. The connectors144 can be configured to carry high-voltage signals, data, controllogic, etc., and can include a single conductor or multipleseparately-insulated conductors. Additionally, where a voltagepotential, control signal, feedback signal, etc., is transmitted viaintervening circuits or structures, such as, for example, for thepurpose of amplification, detection, modification, filtration,rectification, etc., such intervening structures are considered to beincorporated as part of the respective connector. Where other methods ofsignal or data transmission are used, such as via, e.g., fiber optics orwireless systems, such alternative structures are considered to beequivalent to the connectors 144 depicted here.

The abstract of the present disclosure is provided as a brief outline ofsome of the principles of the invention according to one embodiment, andis not intended as a complete or definitive description of anyembodiment thereof, nor should it be relied upon to define terms used inthe specification or claims. The abstract does not limit the scope ofthe claims.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments are contemplated. The various aspects andembodiments disclosed herein are for purposes of illustration and arenot intended to be limiting, with the true scope and spirit beingindicated by the following claims.

What is claimed is:
 1. A method, comprising: performing a burner startupprocedure, including applying thermal energy to a primary flame holderby holding a flame supported by fuel emitted from a primary nozzle at alocation between the primary nozzle and a side of the primary flameholder facing the primary nozzle; terminating the burner startupprocedure after a temperature of a portion of the primary flame holderis above a startup temperature threshold; emitting a flow of fuel fromthe primary nozzle at an operational rate; and supporting a flame withina plurality of apertures extending through the primary flame holder. 2.The method of claim 1, wherein performing the burner startup procedurefurther includes generating turbulence in the fuel emitted from theprimary nozzle by positioning a secondary flame holder between theprimary nozzle and the primary flame holder.
 3. The method of claim 2,wherein terminating the burner startup procedure includes removing thesecondary flame holder from between the primary nozzle and the primaryflame holder.
 4. The method of claim 1, wherein performing the burnerstartup procedure further includes holding the primary nozzle in astartup position and emitting fuel from the primary nozzle at a startuprate, the startup rate being sufficiently low as to enable a stableflame within the fuel flow, and wherein the terminating the burnerstartup procedure includes moving the primary nozzle from the startupposition to an operational position.
 5. The method of claim 1, whereinperforming the burner startup procedure further includes holding theprimary flame holder in a startup position and emitting a flow of fuelfrom the primary nozzle at a startup rate, the startup rate beingsufficiently low as to enable a stable flame within the fuel flow, andwherein the terminating the burner startup procedure includes moving theprimary flame holder from the startup position to an operationalposition.
 6. The method of claim 1, wherein terminating the burnerstartup procedure includes reducing a concentration of oxygen in a flowof oxidizer fluid.
 7. The method of claim 1, wherein terminating theburner startup procedure includes reducing a flame propagation speed inthe flow of fuel.
 8. The method of claim 1, wherein applying thermalenergy to the primary flame holder includes applying an electricalpotential across first and second electrodes positioned to applyelectrical energy to the flame supported by the primary nozzle.
 9. Themethod of claim 1, wherein applying thermal energy to the primary flameholder includes: applying an electrical charge having a first polarityto the flame supported by the primary nozzle; and holding a portion ofthe flame near the primary flame holder by applying a voltage having asecond polarity, opposite the first polarity, to an electrode.
 10. Themethod of claim 9, wherein the electrode is positioned on a face of theprimary flame holder.
 11. The method of claim 10, wherein theterminating the burner startup procedure includes retracting theelectrode from a position between the primary nozzle and the primaryflame holder.
 12. A system, comprising: an oxidant source configured tooutput air into a combustion volume; a primary fuel nozzle configured tooutput fuel into the combustion volume; a primary flame holder having:an input face proximal to the primary fuel nozzle; an output face distalfrom the primary fuel nozzle; and a plurality of apertures extendingbetween the input and output faces; and a preheat mechanism configuredto support a flame in a first position selected to preheat the primaryflame holder, the preheat mechanism being configured to remove the flamefrom the first position after preheating the primary flame holder, theprimary flame holder being configured to support a combustion reactionof the fuel and oxidant within the plurality of apertures after beingpreheated by the first flame.
 13. The system of claim 12, wherein thefirst position is between the primary fuel nozzle and the primary flameholder.
 14. The system of claim 13, wherein the preheat mechanismincludes an actuator coupled to the primary fuel nozzle and configuredto move the fuel nozzle between a preheat position and an operatingposition.
 15. The system of claim 14, wherein the primary fuel nozzleoutputs a preheat flow of the fuel for the flame in the first positionwhen the primary fuel nozzle is in the preheat position and wherein theprimary fuel nozzle outputs an operating flow of the fuel for thecombustion reaction supported by the primary flame holder when the fuelnozzle is in the operating position.
 16. The system of claim 13, whereinthe preheating mechanism includes a secondary flame holder configured tohold the flame in the first position.
 17. The system of claim 16,wherein the preheating mechanism is configured to support the flame inthe first position by moving the secondary flame holder to a preheatposition and to remove the flame from the first position by moving thesecondary flame holder from the preheat position.
 18. The system ofclaim 17, wherein the preheating mechanism is configured to support theflame in the first position by applying a selected voltage between theflame and the secondary flame holder.
 19. The system of claim 18,wherein the preheating mechanism is configured to remove the flame fromthe first position by terminating the flame.
 20. The system of claim 12,wherein the preheating mechanism is configured to remove the flame fromthe first position by moving the flame to a second position.
 21. Amethod comprising: preheating a primary flame holder by supporting aflame at a first position between a primary fuel nozzle and the primaryflame holder; removing the flame from the first position after theprimary flame holder has been preheated to a threshold temperature;outputting fuel from the primary fuel nozzle onto the primary flameholder; and supporting a combustion reaction of the fuel within theprimary flame holder after the primary flame holder has been preheatedto the threshold temperature.
 22. The method of claim 21, whereinremoving the flame from the first position includes terminating theflame.
 23. The method of claim 22, wherein supporting the flame at thefirst position includes holding the flame with a secondary flame holder.24. The method of claim 21, wherein the preheating mechanism isconfigured to support the flame in the first position by moving thesecondary flame holder to a preheat position and to remove the flamefrom the first position by moving the secondary flame holder from thepreheat position.
 25. The method of claim 21, comprising: supporting theflame in the first position by moving the primary fuel nozzle to apreheat position and outputting from the fuel nozzle a preheat flow ofthe fuel for the flame in the first position; and outputting fuel fromthe primary fuel nozzle onto the primary flame holder by moving theprimary fuel nozzle from the preheat position to an operating positionand outputting from the fuel nozzle an operating flow of the fuel.